1. Field of the Invention
The present invention relates to new binder compositions based on copolymers containing epoxide and hydroxyl groups and also organic compounds which contain at least two cyclic carboxylic acid anhydride groups per molecule, and to their use in coating and sealing compositions.
2. Description of the Prior Art
The use of two-component systems based on organic compounds containing at least two intramolecular carboxylic acid anhydride groups per molecule and polyhydroxy compounds as binders for surface coatings and coating agents is known.
FR-A-2,392,092 describes coating compositions based on polymers containing hydroxyl groups and an ester-anhydride containing at least two anhydride groups per molecule. The ester-anhydride is a derivative of trimellitic anhydride.
EP-A-48,128 describes coating compositions based on a hydroxyl component containing at least two hydroxyl groups per molecule, an anhydride component containing at least two anhydride groups per molecule and if appropriate an amine component which acts as a catalyst. A disadvantage of these compositions is that their resistance to solvents is inadequate for high-quality top coats.
Both binder compositions which harden at room temperature and compositions which contain stoving binders based on carboxyl groups and epoxide groups are also already known.
DE-A-2,635,177 and DE-A-2,728,459 describe low-solvent stoving coating compositions based on a polyacrylate containing carboxyl groups, an epoxy resin containing at least two epoxide groups per molecule and a solvent mixture. The slowly progressing preliminary reaction between free carboxyl and free epoxide groups has an adverse effect in these systems and may result in an inadequate pot life for the coating compositions.
International patent application WO 84/00768 describes ternary thermosetting coating compositions based on (i) a polyacrylate containing epoxide and hydroxyl groups, (ii) a specific dicarboxylic acid anhydride and (iii) an amine resin cross linking agent. However, the anhydride component contains only one carboxylic acid anhydride group per molecule. As can be seen from the examples, coating agents based on these ternary systems can be hardened only at high hardening temperatures, for example about 130.degree. C., even in the presence of catalysts. International patent applications WO 84/00770 and WO 84/00771 describe similar systems, with the difference that the first component contains only hydroxyl groups and a separate component containing epoxide groups is additionally also present.
The use of three-component systems based on organic compounds containing at least two intramolecular carboxylic acid anhydride groups per molecule, polyhydroxy compounds and polyepoxide compounds as binders for surface coatings and coating agents is also known. These systems are described in EP-A-134,691, EP-A-316,874 and EP-A-358,306.
Although the multi-component systems mentioned result in coatings with improved solvent resistance when compared to the above-mentioned two-component systems, they have the disadvantage that the individual components must be completely compatible with one another in order to ensure a good optical appearance of the coating. This compatibility problem limits the range of variation of the individual components.
An object of the present invention is to provide new binder compositions based on organic compounds containing carboxylic acid anhydride, hydroxyl and epoxide groups, which do not have the disadvantages mentioned for the systems of the prior art and in particular have a pot life which is adequate for surface coatings. It is an additional object of the present invention to provide binder compositions which, depending on the field of use, can be hardened at room temperature or elevated temperature to give clear, colorless, yellowing-resistant and solvent-resistant surface coatings.
These objects may be achieved with the binder compositions of the present invention which are described below in more detail.
A component A) which contains both free epoxide groups and free hydroxyl groups and a component B) which contains at least two cyclic carboxylic acid anhydride groups per molecule are present in the binder compositions according to the invention. Such binder compositions have not previously been described. The particular advantage of such systems lies in the fact that they contain the reactive groupings in the form of epoxide, hydroxyl and carboxylic acid anhydride groups in only two components A) and B), and that initially no free carboxyl groups, which are required for crosslinking with the epoxide groups of component A), are present in component B). These carboxyl groups are produced only from the reaction between the carboxylic acid anhydride groups and the hydroxyl groups in the presence of catalysts or heat.
These binder compositions thus have a considerably better storage stability at room temperature than comparable systems of the prior art which contain free carboxyl groups. Another advantage of the binder compositions according to the invention is that the crosslinking reaction is a so-called "double-cure" mechanism, i.e., the hydroxyl groups of component A) first react with the anhydride groups of component B) to open the anhydride ring and form carboxyl groups. This reaction provides initial crosslinking. However, the newly formed carboxyl groups can also react with the epoxide groups of component A) to provide additional crosslinking.
Coatings which have very high resistance to chemicals and solvents result from the binder compositions according to the invention due to the high crosslinking densities which can be obtained.